This invention relates to a novel type of negative-resistance solid state element, and more particularly to such type of element that exhibits negative differential conductivity upon application thereto of a high electric field and to applications thereof.
Heretofore, negative-resistance elements such as the so-called Exaki diode, which utilize a tunnel effect of semiconductors, have been known. However, in the Exaki diode, since the negative resistance is obtained at the P-N junction of the semiconductive substances, the negative differential conductivity is exhibited only for a specific polarity, and because of the capacitance existing at the junction point, the element cannot be used at frequencies higher than 10 GHz. These features together with its limited output power constitute the drawbacks of this type of diode.
The so-called Gunn diode is also known, wherein a semiconductive material such as GaAs which has two valleys in its conduction band is employed. When a high electric field is applied across such an element, electrons are transferred from the lower energy valley to the higher energy valley, and because mobility of the electrons in the higher energy valley is less than the mobility in the lower energy valley, the average speed of electrons decreases with increase in electric field. When the intensity of the internal electric field applied from outside as described above exceeds a critical value (about 3000V/cm), a high field domain is created near the cathode, which is thereafter shifted to the anode by the action of the applied electric field. When the high field domain reaches the anode, it disappears at once, and an impulsive current is caused to flow through the semiconductive substance because of the disappearance of the high field domain. Following this disappearance, new high field domain is created near the cathode and the same sequences mentioned above are repeated at a frequency determined by the length of the element.
This type of solid state element can be utilized in generating high-frequency oscillation of a frequency determined by l/v.sub.d, wherein l designates the length of the element, and v.sub.d designates the velocity of the high field domain. Considering the fact that the velocity v.sub.d of the high field domain is about 10.sup.7 cm/sec., it is apparent from this formula that the length of the element must be minimized to an extremely short value (of the order of several microns) if it is desired to obtain microwave or millimeter wave-length.
Despite various efforts to obtain still higher frequencies than those described above, it has been found that the practical limitation exists around several tens of GHz, and the resultant oscillation output is rapidly decreased with increase in the frequency. Such features together with its excessively narrow frequency band constitute drawbacks of the Gunn type solid element.
The so-called LSA diode is also known. It is believed that with this type of diode further increase in the oscillation frequency can be attained with a moderate efficiency. However, in this case, since the biasing electric field must be higher than twice the value of the Gunn diode, the semiconductive material must be of extremely uniform quality, and, moreover, as there is a limitation in the relationship between the electron density and frequency, these are other features constituting the shortcomings of the LSA diode.
It has been found that, if one or whole part of the surface of a semiconductive element of, for instance, GaAs, is covered by a dielectric layer or a metallic layer which is reactively coupled with the semiconductive element through an intermediate dielectric thin layer, the occurrence of the high field domain at the time when a high electric field is applied thereacross can be prevented, and a phenomenon which might be called "negative-differential resistance characteristic" or "negative differential conductivity" can be obtained. Semiconductors exhibiting this phenomenon are described as exhibiting "negative differential conductivity" or "negative differential mobility" in their drift velocity electric field characteristic.